This invention relates, generally, to a resonant circuit oscillator for a metal proximity sensor and, particularly, to a proximity oscillator of the type which produces an output with a peak amplitude that varies, as a function of target distance, the distance of a metal body or target from a probe containing a part of the resonant circuit.
Proximity sensors are known which utilize a resonant circuit oscillator having an output amplitude that varies in accordance with the distance between a conductive body and a probe containing a part of the resonant circuit. In such sensors, a level detector provides an output indication in response to the output oscillation assuming an amplitude corresponding to a selected distance.
Feedback oscillators of the Hartley or Colpitts type are commonly used which have an inductance coil of an LC resonant circuit contained in a probe. The transfer function of the feedback portion of the oscillator is established, in part, by the effective load impedance Q and power loss of the resonant circuit including the inductance coil of the probe. Within a certain range of distance from the probe, commonly less than an inch, eddy current losses in a metal body or target load the resonant circuit. This loading can be analytically expressed in terms of the effective parallel circuit resistance which varies directly with target distance or the effective equivalent series resonant circuit resistance which varies inversely with distance.
Movement of the target toward the probe decreases the effective parallel resistance and increases the power loss of the resonant circuit. Likewise, as the distance between the probe and the target increases, the effective parallel resistance of the resonant circuit increases and the power loss decreases.
In these feedback oscillators, the relationship between resonant circuit power loss and oscillator output amplitude is substantially nonlinear. For a range of relatively short distances between the probe and the target, the power losses of the resonant circuit are so high that the feedback loop gain is too low to sustain oscillations. Over this range of relatively short distances, changes in the resonant circuit power loss have virtually no effect on the output. When the power loss decreases to a value at which the loop gain is sufficiently high so that the oscillator amplifier enters its active region, oscillation amplitude increases with decrease in power loss. This relationship continues through the active region of the amplifier. Through a small portion of the active region of the amplifier, oscillator amplitude is linearly related to the inverse of power loss. Upon the power loss decreasing to the point where the amplifier reaches saturation, the peak output amplitude is achieved and further decreases in power loss have no effect. The nonlinearity of the oscillator output is due primarily to its feedback operation.
Nonlinearity of proximity sensors is due primarily to the oscillator, and not to probe characteristics. Many sensing probes have a linear relationship between the inverse of the power loss (or the effective resistance) and distance from a conductive body for a distance range which is substantially greater than the linear range of the oscillator. But most of the linear range of the probe cannot be advantageously employed because the oscillator is nonlinear.
The characteristics of feedback oscillators present a number of problems which detract from their utility in proximity sensor applications. The nonlinearity of the oscillator makes difficult calibrated adjustment of the relationship between output magnitude and distance. The feedback operation makes these known oscillators susceptible to temperature instability and mechanical noise problems.
The frequency response of these oscillators is also severely limited. The gain of the oscillator amplifier must often be kept low to maximize the linear range, and this degrades the frequency response of the oscillator. Oscillators of the Hartley and Colpitts type are easily loaded to the point where they cease to oscillate completely, and this further degrades the response time thereof.
Further, prior art sensors with oscillators of this type typically use as detectors level comparators with large amounts of hysteresis to cover up the instabilities in the oscillator. The wide excursions in oscillator voltage required for a level comparator with large amounts of hysteresis still further degrade the response time of the sensor.